1. Field of the Invention
This invention relates in general to treating oil and gas wells with carbon dioxide, and in particular to an improved method for injecting the carbon dioxide into a well.
2. Description of the Prior Art
In order to increase production, oil and gas wells may be hydraulically fractured or treated with other chemicals. In the "frac" treatment, a liquid slurry under high pressure is pumped down the well to create cracks in the formation. Glass beads, grains of sand, or the like, in the slurry enter the cracks to prop them open for improved formation flow.
One fluid that has been mixed with the slurry in the past is carbon dioxide. Carbon dioxide has several advantages, such as reducing the swelling of clays, lowering the pH of the treating fluid, and also helping in removing blocks from the formation. Carbon dioxide is also useful in treating the well with other chemicals such as acids. Normally the carbon dioxide is pumped in as a liquid. Once it exceeds its vaporizing temperature at the injection pressure, it begins changing into a vapor state, or gas. Once the carbon dioxide exceeds its critical temperature it becomes a gas. The volume increases dramatically during the change in phase. The carbon dioxide enters the perforations in the formation. The vapor forms a gas lift for lifting the other liquids out of the well after the surface pressure is removed.
One difficulty occurs in obtaining sufficiently high flow rates of carbon dioxide to mix with the frac slurry being pumped down at high rates. Carbon dioxide is brought to the well site in a tank truck. The temperature is about 0.degree. F. (Fahrenheit), and the pressure is around 300 psig (pounds per square inch gage). Under these conditions, the carbon dioxide is in a liquid state, but for a small vapor blanket at the top of the tank. The liquid is drawn off to a booster pump which pumps the liquid to a level about 50 to 125 psig above the tank pressure. The purpose of the booster pump is to reliquefy partially vaporized carbon dioxide that vaporises in the discharge line due to the pressure drop. However, as the tank pressure drops due to withdrawal, more and more of the liquid vaporizes in the line. The vapor blanket increases in size, but the liquid in the tank does not vaporize sufficiently fast to maintain tank pressure. The very fast vaporization in the line, known as "flashing" absorbs the latent heat of vaporization, thus lowering the temperature drastically. The temperature may drop below the freezing point of carbon dioxide, freezing the liquid carbon dioxide and blocking the line. Also a large amount of vapor may cause the booster pump to vapor lock. Consequently, only small flow rates of carbon dioxide can be achieved.
The pumping of the liquid slurry during a frac treatment cannot be stopped until the fracturing is completed. Otherwise, the well will "sand up" due to the large amount of sand in the slurry enroute to the perforations. Consequently, if the carbon dioxide booster pump vapor locks, or if the line to the booster pump freezes, the carbon dioxide will have to be deleted from the rest of the treatment.
Also, there are certain instances in which pumping of vaporized carbon dioxide from the surface would be useful, such as in low temperature wells. However, because of the low flow rates available from liquid carbon dioxide transport tanks, sufficiently high flow rates for treatment with carbon dioxide vapor are not available.